As illustrated in FIG. 13, there has been conventionally known an injection molding method (see Japanese Patent No. 3383971) that injects a molten resin from a pinpoint gate 1000 into a cavity 1002 in a mold 1001 to manufacture a cylindrical article 1003 (see FIG. 14) to which a shape of the cavity 1002 is transferred.
However, as illustrated in FIG. 13 and FIG. 14, after the molten resin is injected from the pinpoint gate 1000 into the cavity 1002 in the mold 1001, the conventional injection molding method forms a weld line 1004 at a part where the molten resins join together in the cavity 1002. A problem therefore occurs in that this weld line 1004 deteriorates strength of the cylindrical article 1003, as has been pointed out. Especially, in the cylindrical article 1003 on which the injection molding has been performed using a fiber-reinforced resin material, reinforcing fibers 1005 at the weld line 1004 are arranged in one direction (molten resin flowing direction) (see FIG. 15). This has a problem that press-fitting the cylindrical article 1003 to a shaft is likely to cause a crack at a part where the weld line 1004 is formed.
Therefore, an object of the present invention is to provide a cylindrical article made of a fiber-reinforced resin material, an injection molding mold thereof, and an injection molding method to improve strength of a part where a weld line is formed.